Constraining the Age and Evolution of an Axial Volcanic Ridge on the Mid Atlantic Ridge

Axial
volcanic ridges (AVRs) are a common feature found within the median valley of
slow spreading ridges such as the Mid-Atlantic Ridge (MAR). They are sinuous volcanic ridges tens of
kilometers long, a few kilometers wide and few hundreds of meters high and
comprised of a variety of volcanic products varying from pillow lava benches to
hummocky lava to isolated cones, lava terraces and lava lakes. Occurring in over 90% of MAR ridge segments
surveyed to date, AVRs are widely recognized as the fundamental volcanic
accretionary “building block” in slowly spread crust. Despite this morphological characterization,
very little is known about the age and genetic relationships between the
various volcanic products that make up an AVR or the age span and episodicity
of the volcanic processes involved in its construction. In fact, there are no quantitative age dates
of any volcanic products from the median valley of the Mid-Atlantic Ridge. Part of the reason for this has been the lack
of dating techniques to address the age ranges involved (<300000 yrs),
however, this has now been remedied by the development of U-series disequilibria
which has been successfully applied in a variety of environments. Another reason has been the lack of precise
geological context for the samples with maps of sufficient spatial resolution
to document sample locations in relation to the various volcanic units. This too has been remedied with the use of
near-bottom multibeam bathymetric and optical mapping that provide not only
sub-meter resolution but also extend such coverage over several kilometers. High-resolution magnetic mapping has also
benefited from this scale of mapping and we have found that rapid changes in
geomagnetic field intensity (as opposed to polarity reversals) also generate
magnetic anomalies that allow us to provide chronological markers as well as
infer patterns of crustal accretion.

With these
tools and expertise resident here at WHOI we feel the time is ripe to take an
important step forward in our knowledge of volcanic emplacement at the slow
spreading MAR. We therefore propose to
provide the first quantitative age constraints on midocean ridge basalts from
the MAR and thereby constrain the evolutionary history of an AVR. We seek funds to participate in a British
research expedition that will investigate the history, genesis and evolution of
an AVR on the MAR at 45°N using the remotely-operated vehicle (ROV) ISIS and deep
towed sidescan sonar vehicle, TOBI. The
British team is led by Prof. Roger
Searle of DurhamUniversity with a
primarily geochemical science party focused on issues related to the evolution
of magmas and melting processes with respect to an AVR. Our contribution will be to provide age
constraints using high-resolution magnetic chronology with sensors mounted to
TOBI and ISIS and absolute age-dating of samples
collected by ROV. The cruise is
scheduled for 2008 on board RRS James Cook.These data will be important as they will not only provide quantitative
age constraints necessary for understanding the evolution of a slow spreading
midocean ridge but also how volcanic ages are related to hydrologic and biologic
systems. While we will measure only 5-10
samples using these funds, these preliminary data will provide
‘proof-of-concept’ necessary to obtain NSF funding for a more complete study.

Originally published: May 16, 2007

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